An elderly New Zealand man who suffered a heart attack at a hardware store was revived by a salesman who just happened to be demonstrating a defibrillator to store staff.

The man, who was in his 80s, collapsed in an aisle of the Placemakers store in Albany, near Auckland, and his heart was not beating when salesman Gavin MacDonell attached the machine to his chest.

It was the ultimate sales pitch demonstration for Mr MacDonell, who has been a St John Ambulance volunteer paramedic for 20 years.

"Once we got a shock into him he started to gag, and we thought 'this bloke's coming back'," he told the New Zealand Herald newspaper.

"The people in the shop that were working there were just blown away."

The victim was expected to recover, the paper said.